Patent Literature 1 proposes an intercom system over a network as a communication system in the premises. In this intercom system, a plurality of terminal devices (slave devices) belong to a group so as to enable simultaneous mutual talk in the group. That is, the plurality of terminal devices connected to an access point (master device) connected to a main control device are grouped into a plurality of groups, and terminal devices in each group perform simultaneous mutual talks, respectively.
In this intercom system, terminal devices belonging to a certain group can change a group to which they belong.